A single-point spectrometry measurement is generally sufficient for analysing a spatially homogeneous sample. However, in the case of a spatially inhomogeneous sample, it is desirable to perform spectrometry measurements in the form of an image constructed point by point with a spatial resolution adapted as a function of the structure or the microstructures of the sample.
Different techniques of spectrometry imaging are known, based on the point-by-point acquisition of spectra and on the reconstruction of a spatially resolved spectrometry image. These systems require a displacement of the sample with respect to the measurement instrument and to an excitation beam focused to one point, or a scanning of the excitation beam with respect to the sample, or also a combination of a beam scanning and a displacement of the sample.
In a system of Raman microspectrometry imaging based on a displacement of the sample, the sample is placed on a plate that is motor-driven or provided with piezoelectric actuators, to displace the sample along two transverse directions (XY). The laser excitation beam is focused to the sample, in an area of about 1 micrometer in diameter. The displacement of the sample generally follows a periodic grid having a constant pitch along X and along Y. A spectrum is acquired at each new position of the sample. A processing software reconstructs a hyperspectral image from this set of measurements.
It is also known from the patent document EP1983332A a method of spectroscopy imaging and a system for scanning exploration of the surface of a fixed sample. The document EP1983332A describes a spectroscopy imaging apparatus comprising a scanning device, also called a scanner, for exploring the surface of a fixed sample by angular displacement of a laser excitation beam along two transverse directions. More precisely, the document EP1983332A describes a scanning device placed in the tube of a confocal microscope so as to be inserted between the microscope lens and the injection-rejection filter of a Raman spectrometer. The scanning device operates in one direction to angularly displace the laser excitation beam so as to position it at different points of the surface of the sample. By reverse return of light, this scanning device operates in the reverse direction to collect a Raman back-scattering beam and to transmit it to a detection system, for example a Raman spectrometer. This apparatus makes it possible to acquire point by point a Raman spectrometry image of a portion of the surface of a sample with a resolution of about 50×50 points within about 10 minutes. A change of magnification of the microscope lens makes it possible to modify the extent of the scanned surface on the sample.
Other patent documents describe beam scanning microspectrometry imaging apparatuses (see for example WO 2010/069987, US 2005/128476 or JP 2001 091848).
Whatever the type of displacement chosen, the duration of point-by-point acquisition of an image is determined by the duration of acquisition of each point and the number of points of the image. The duration of acquisition of each point is linked to the light and to the spectral resolution of each point. The number of points of the image depends on the pitch of displacement of the sample with respect to the excitation beam and of this beam with respect to the sample, which determines the spatial resolution of the image.
The acquisition of a single Raman spectrum generally takes between 0.1 ms and 1 minute, and on average 1 second. To obtain an image with a high spatial and spectral resolution, the duration of acquisition of a Raman microspectrometry image can reach several hours or even several tens of hours. These durations are not adapted for the analysis of several samples.
The main options to reduce the duration of acquisition of spectrometry images are reducing the duration of acquisition of each point and reducing the number of points in one image. However, these options generally lead to a loss of quality of the reconstructed image, a diminution of the signal to noise ratio of each point and/or a diminution of the spatial resolution of the reconstructed image. In certain applications, it is desirable to reduce the duration of acquisition of a microspectrometry image or a hyperspectral image without loss of luminosity, of signal to noise ratio, of spectral resolution nor of spatial resolution in the image reconstructed point by point.
In other applications, it is desirable to increase the spatial resolution of a hyperspectral image or a microspectrometry image without increasing the duration of acquisition of this image.